Why is blenders hair system making spikes?
The setting for hair particles in cycles are in Particles > Cycles Hair Rendering and Particles > Cycles Hair Settings.

To make the hairs a constant thickness, set the root and tip thickness settings to the same value and disable Close Tip:



To get a cylinder-like result, you will also have to disable Particles > Cycles Hair Rendering > Cull Back-faces so the backs of faces are not hidden:



See the manual for more information.